1. Field of the Invention
This invention relates to a positioning control method and device for positioning the head of the disk device at a target position, and more particularly to a positioning control method and device for compensating for non-linear detection characteristics of the head.
2. Description of the related Art
Disk devices that read a disk medium with a head are widely used. For example, magnetic disk drives that are used as a storage device for a computer comprise a magnetic disk, a spindle motor for rotating the magnetic disk, a magnetic head for reading from and writing to the magnetic disk, and a VCM actuator for positioning the magnetic heat at a track on the magnetic disk. The recording density of this kind of disk drive is rapidly increasing, as well as is the track density of the magnetic disk. Particularly, by using an MR head for the magnetic head, high density storage have become possible. Therefore, high-precision positioning at high speed is becoming necessary.
When the disk drive receives a read or write command from the computer, it moves the magnetic head from the current position to the target position. This is called the seek operation. This seek operation is a transition operation which moves to following control after coarse control.
Feedback control is used for moving to the target position and for following at the target position. Feedback control detects the current position of the head, calculates the position error between the target position and the current position, and controls the actuator for the head in order to do away with the position error.
In order to detect the current position of the head, position information is recorded on the disk. The head reads the position information from the disk and detects the current position from the position information that is read. In addition, it calculates the position error between the target position and the current position. In the disk device, the current position is demodulated from a position signal that is read by the head, so the detection characteristics of the head affect the accuracy of detecting the current position. Therefore, it is necessary to compensate for the detection characteristics.
FIG. 22 is a schematic drawing of the feedback control system of the prior art. The position information (servo information) of the magnetic disk comprises a track number and a servo (burst servo) signal. The servo signal, for example, is a 4-phase servo signal that is 90-degrees out of phase. The position information that is read by the head is demodulated by a demodulation circuit, to obtain the track number and offset signal. The offset signal is obtained from the amplitude of the servo signal, and its magnitude is proportional to the amount that it has shifted from the center of the track for that track number.
The output level of the magnitude of this offset signal changes according to the characteristics of the magnetic head and the track position. Therefore, it is necessary to correct this change, and to convert the value of the offset signal to a value in actual track units. A method for doing this is known (for example, as disclosed in Japanese Unexamined published Patent No. H8-195044) where gains for the magnetic head and each track position (called the position sensitivity) are set, and the offset value is corrected by referencing the gain that corresponds to the head and track position.
In this method, it is assumed that the detection sensitivity of the head is uniform within the width of the track. However, as the track width becomes narrow, it is not possible to assume that the detection sensitivity of the head is uniform within the same track, due to the core width of the head. Especially, there is a strong tendency for this in the case of a MR head or GMR head. Therefore, it is not possible to accurately convert the demodulated offset value to the offset value of the track position. This error causes fluctuation in the loop gain of the feedback control system, and reduces the positioning accuracy.
A first method for correcting the non-linear characteristics of the head has been proposed (for example, as disclosed in Japanese Unexamined published Patent No. H10-222942). In other words, as shown in FIG. 22, there is a correction table 90 for storing the conversion gains (called position sensitivity) for the head, and each track position and offset position. The correction table 90 is referenced according to the demodulated offset value, in order obtain the corresponding correction gain. For example, as shown in FIG. 24, the correction table 90 stores correction gains (for example, for each 0.05 track) for each offset position indicated by a real position (Real Position).
A multiplier 91 multiplies the demodulated offset value by the correction gain to obtain a decoded offset position. As shown by the dotted line in FIG. 25, when this gain is a suitable gain, the decoded offset position indicates the real offset position. Furthermore, by adding this offset position and the track number with an adder 92, the current position is obtained. In addition, a computing element 93 subtracts the target position from the current position to calculate the position error. The position error is input to a servo-control unit 94. As mentioned above, the servo-control unit 94, performs coerce control, integral control or following control, depending on the value of the position error, and outputs the control amount. The head actuator (not shown in the figure) is driven according to this control amount, to position the head at the target position (target track).
On the other hand, another non-linear correction method, as shown in FIG. 23, is known (for example, Japanese Unexamined published patent H8-249844). In FIG. 24, the loop gain 97 of the controller 94 can be changed. Also, the gain of a demodulation circuit (position detection circuit) 96 for the each offset position from the track center is measured by a measurement circuit 98, and stored in memory. The gain 97 of the controller 94 is controlled by referencing the gain of the measurement circuit 98 according to the demodulated current position.
In this method, the position error is not corrected, however, by controlling the gain of the controller 94, the fluctuation of the loop gain of the control system due to the non-linearity of the head output is suppressed, and stable positioning is performed even at the offset position.
However, the prior art had the following problems.
(1) In the first non-linear correction method, it is difficult to obtain an accurate correction gain (correction curve) from the correction table 90. In other words, it is necessary to find the correction gain with the absolute position as a reference. Therefore, the correction gain for the absolute position is measured by the STW (servo track writer). However, the value measured by the STW is not necessarily the optimum value for each drive (device). In other words, the detection sensitivity changes due to the solid difference or setting of the demodulation circuit. Therefore, the relationship between the decoded position and the real position shifts, as shown by the solid line in FIG. 26, even though the gain measured by the STW is used, and this makes it impossible to correct to the accurate offset position for each individual drive.
(2) Also, the detection sensitivity of the head changes due to wear of the core end of the head or changes in characteristics of the MR element, and there is no guarantee that the detection sensitivity of the head will not be permanently changed. For this reason as well, there is the problem that it is not possible to correct to the accurate offset position for each individual drive. To solve this problem, it is necessary to measure the correction gain for the drive itself, however, since the drive controls positioning by using the position distorted by the effect of the head sensitivity distribution, it is difficult to find the accurate absolute position by using the distorted position, and there is the problem that the drive cannot measure the correction curve for the absolute position.
(3) In the second non-linear correction method, non-linear correction is performed by controlling the loop gain, so the control system is not affected by the non-linear characteristics, and it is possible to prevent a drop in the positioning accuracy. However, it is not possible to obtain accurate position error. Therefore, when allowing read/write from the position error, it is not possible to obtain accurate read/write timing. For example, there is about 20% error. Therefore, the start of reading or writing is slow, and the time it takes to start reading or writing after receiving a command becomes long and there is a drop in performance.
(4) Moreover, in the second non-linear correction method, since accurate position error cannot be found, the control system that is suitable for the controller 94 is limited to a lead-lag filter or PID control filter. For example, there is a problem in that an observer control system that requires accurate position error, is not suitable for the controller 94.
The objective of this invention is to provide a head-positioning control method and device for correcting the non-linear characteristics of the head and detecting accurate position error.
Another objective of this invention is to provide a head-positioning control method and device for detecting accurate position error even when using correction gain that is measured by the device itself.
A further objective of this invention is to provide a head-positioning control method and device for accurately detecting the starting point for reading or reading and writing.
Yet a further objective of this invention is to provide a head-positioning control method and device for which the control system of the controller is not limited even when correcting the non-linear characteristics of the head.
In order to accomplish the objectives of this invention, one form of the head-positioning control method for a disk device of this invention comprises: a step of demodulating a position information on a disk that is read by a head; a step of calculating a position error between the demodulated position and the target position; a step of correcting the position error using a set position gain according to an offset from a center of the track of the target position; a step of calculating the amount of control for the actuator that drives the head according to said corrected position error. Furthermore, said method includes a step of generating permission for the head to read or write, according to said corrected position error.
Moreover, one form of the head-positioning control device for a disk device of this invention comprises; a demodulation circuit for demodulating the position information on the aforementioned disk that is read by the head; and a control circuit for calculating the amount of control for the aforementioned actuator according to the position error between the aforementioned demodulated position and the target position; wherein the control circuit corrects the position error by a set position gain according to the offset from the center of the track of the target position, then calculates the amount control for the actuator that drives the aforementioned head according to the corrected position error. Furthermore, the control circuit generates permission for the head to read or write according to the corrected position error.
In this form of the invention, first, correction is performed using the distorted position without using the absolute position. Therefore, it is necessary to separate the offset and the position gain. In other words, in the prior first non-linear correction method, the absolute position from the STW is used as a reference and the it is necessary for the system to find the offset value (absolute value) from that, therefore it is not possible to separate the offset and the gain. However, in this invention, a gain is used for the position error between the demodulated offset value and the target value. In other words, gains are set for the target position and for a relative offset value. In this way, the absolute position does not need to be used, and since it is performed according to the relative position, it is possible to accurately obtain the position error even when using the distorted position.
Second, in the prior second non-linear correction method, the offset and gain are separated, however, since the loop gain is corrected, it is not possible to obtain the correct position error. In this invention, it is possible to obtain the correct position error, so it is possible to accurately detect the start of reading or writing. Moreover, it is possible to a control system, such as observer control, as the control system for the controller.
In the head-positioning control method of another form of the invention, wherein the correction step comprises: a step of using the aforementioned position error as the correction position error during seek control, and a step of correcting the aforementioned position error by the aforementioned position gain during following control.
In this form of the invention, correction is performed with the position gain during following, and gain correction is not performed during seeking. When a position gain is set for each offset position, since the head is continuously moving during seek control, the position gain that is referenced by the target position does not indicate the proper gain during operation. Therefore, seek control is not performed smoothly with position gain. To prevent this, correction by the position gain is prohibited during seeking.
In the head-positioning control method of another form of the invention, the aforementioned demodulation step comprises a step of correcting the offset signal of the aforementioned position information by the sensitivity gain that is set for the track number in the position information.
In this form of the invention, the continuity of the offset position is obtained from the phase-servo signal during demodulation, so correction is performed with the sensitivity gain.
The head-positioning control method of yet another form of the invention, further comprises a step of adding a sine wave to change the aforementioned offset position, and measure the values for the position gains at each offset position.
In the head-positioning control device of another form of the invention, the aforementioned control circuit adds a sine wave to change the aforementioned offset position, and measures the values for the position gains at each offset position.
In this form of the invention, first, the position gains for the device itself are measured. Therefore, it is possible to obtain the position gains that correspond to the characteristics of the device and head. Second, since measurement is performed by adding an external disturbance or sine wave, it is possible for the device to easily measure the position gain (slope).